The United States is the world's largest producer of corn. U.S. production reached 9.5 billion bushels in 2001, greatly exceeding the production of any other grain (National Corn Growers Association 2002). Direct use as animal feed is the largest consuming application, accounting for 5.85 billion bushels in 2001. Corn processing, either via wet milling into sweeteners, starch, ethanol and other industrial products, or via dry milling for ethanol production, accounted for 1.7 billion bushels of US consumption in 2001, or slightly less than 18% of the crop.
Corn processing is expected to increase significantly over the next decade. Ethanol production has been the largest single application of corn processing since 1999, reflecting the recent high growth of the fuel ethanol market as an alternative fuel to gasoline alone. Legislative and lobbying efforts are promoting a renewable fuels standard for gasoline. Most projections are for a three-fold increase in ethanol production, accounting for another 1.4 billion bushels of corn consumption if no significant changes are made to existing manufacturing processes. Ethanol not only burns cleaner than fossil fuels but also can be produced using grains such as corn, which is a renewable resource. Further, the production of ethanol results in new sales outlets for corn, provides additional jobs, and reduces the nation's dependency on foreign oil.
Ethanol is typically produced from corn through either a wet or dry milling process. In the wet milling process, the corn kernel is separated into its components including germ, fiber, protein and starch. These may be further processed into several co-products. For example, separated germ may be further processed for oil recovery; starch may be saccharified and fermented for ethanol production; and protein and fiber may be used as feed material.
In a dry mill process, whole corn is ground into flour, treated with enzymes, and cooked. The resulting “mash” is treated with enzymes to further break down the starchy endosperm tissue into glucose. The converted mash is fermented and distilled, producing ethanol, carbon dioxide, and distiller's grains with solubles (“DDG”). If sold as wet animal feed, the co-product is known as distiller's wet grains with solubles (“DWGS”). Conversely, if dried for animal feed, the co-product is known as distiller's dried grains with solubles (“DDGS”). In the standard dry grind ethanol process, one bushel of corn yields approximately 8.2 kilograms (i.e. approximately 18 pounds) of DDGS in addition to the approximately 10.2 liters (i.e. approximately 2.7 gallons) of ethanol. These co-products provide a critical secondary revenue stream that offsets a portion of the overall cost of ethanol production.
Within typical ethanol production facilities, a small percentage of water, typically 5% by volume, is present in the ethanol resulting in 190-proof ethanol. Current technology for ethanol production employs molecular sieves to remove the last approximately 5% of water from the product. A molecular sieve is typically a bottle containing ceramic beads or other medias with an affinity for water, as known by those of skill in the art. When the 190 proof ethanol passes through a molecular sieve, the molecular sieve pulls the last approximately 5% of water resulting in anhydrous ethanol, i.e. 200-proof, that can be blended with gasoline.
Processes running vacuum distillation systems will typically condense the resulting 190-proof ethanol vapor and then pump it into a vaporizer which in turn is then fed into one or more molecular sieves at a much higher pressure to produce 200-proof ethanol. However, after a sieve bottle is in production for a period of time, the media becomes saturated and may start passing ethanol containing more moisture than is desired. At this point, the molecular sieve will be taken off line and depressurized. Typically the molecular sieve bottle is depressurized back to the rectifier condenser from the feed side of the bottle. This is referred to as reverse flow. The reverse flow stream is then pumped back to the sieve vaporizer as a recycle stream which will use additional energy for the vaporization and pumping of this ethanol.
Reverse flow systems for regenerating molecular sieves reduce plant capacity and requires excess plant energy to run. Because there is a large number of existing ethanol plants based on corn dry milling, and the number is increasing rapidly, it would be desirable to have a process that could be integrated into these plants to improve plant efficiency by freeing up plant capacity while at the same time reducing energy consumption.